Various methods are used in order to protect from forgery or to assure the authenticity of printed documents.
The use of watermarked paper and the impressing of holograms are among those methods which offer a greater guarantee. These are two fairly expensive methods either, in the case of watermarked paper, for the costs involved in the paper manufacturing process, or, in the second case, for the costs of preparation of the type plate (electrotype) and even more of the paper substrate in that said substrate is usually previously coated with a film of a material (such as PVC, polyester, polypropylene, polythene, coating, etc.) suitable to receive and permanently retain the holographic microengravings engraved by the electrotype. Moreover, in the case of holograms the manufacturing costs are economically sustainable only for mass productions (generally using substrate material in reels) whose plant require high investments.
For example, WO-A-89/03760 discloses a method for impressing holograms on a paper or plastic sheeting by providing a coating thereon and then embossing the coated substrate. The coating can be applied in different ways according to the thermoplastic material employed in the process (e.g. polyethylene by extrusion). Regardless of the specific process, this coating step of covering the whole surface of the sheet with a uniform coating layer implies the above-mentioned problems.